Synthetic fungicides are predominantly used for the control of fungi on crops. However many exhibit other toxic effects and could face future removal from the marketplace as controls and regulations governing agricultural chemicals tighten. In other instances, the public trend towards natural products may cause consumer resistance to the use of `perceived` synthetic and non-natural substances.
Another problem of the art is the growing resistance of many targeted organisms substances to commonly used control agents. Accordingly there is a need for further alternatives to the currently used controlling agents, and ideally an alternative to existing control agents to which little resistance is exhibited by fungi and/or microbes.
Armillaria, a fungal pathogen of forest trees, was first identified 115 years ago, and is now recognised as a major problem in a variety of woody plant species world-wide. More than 500 different plant species are known to be susceptible to the organism. In undisturbed forests and native bush the organism rarely causes serious damage, but when trees are harvested, the rotting stumps and roots provide a rich source of nutrients so that the Armillaria may become destructively infective to any remaining shrubs and trees.
Armillaria can be devastating to the forest industry, and billions of dollars are lost annually due to affected timber. The worst losses follow reforestation after clearing the natural tree cover. In New Zealand first time losses from less than 5 to over 90% for Pinus radiata, a major timber crop for local and export markets, have been attributed to Armillaria.
Another major crop affected by Armillaria is kiwifruit, in which the orchards are generally planted on cleared lands. Armillaria was listed as a new disease for kiwifruit in New Zealand in 1955. However, the first detailed account of Armillaria infection in kiwifruit was in a US Department of Agriculture orchard in California; and it described the decline and death of the vines from 1967-1971.
Before 1980, the incidence of Armillaria in New Zealand kiwifruit was only occasional and it was considered to be a minor phytopathogen. Between 1980 and 1990 a dramatic increase occurred in the number of infected orchards, and the industry suffered as a consequence. Significant industry losses are expected if the disease continues to spread.
Both kiwifruit and Pinus radiata are major export crops for New Zealand and treatment of these commodities with synthetic pesticides is unacceptable to many export markets, and the public consumer. Insofar as controlling Armillaria is concerned, Leach in 1936 reported some benefit from ring-barking forest trees; however, this is not advisable in New Zealand because willow trees treated this way appear to have high incidence of Armillaria, and willow is one of the shelter trees used in kiwifruit orchards.
Other plant pathogens are also commercially damaging. For instance Botrytis cinerea is well known for its effects on grapes and its responsibility for afflictions such as kiwifruit storage rot, as well as grain mould of grapes and strawberries, etc. Botrytis diseases are among the most common and most widely distributed diseases of vegetables, ornamentals, fruits and glasshouse crops throughout the world. In New Zealand, Botrytis cinerea attacks many economically important horticultural crops such as kiwifruit, bean and strawberry, and in particular is the causal agent of two important diseases--grey mould of tomato and bunch rot of grape. In the past, control of these diseases has relied extensively on the use of benzimidazole and dicarboximide groups of fungicides. However, the development of fungicide resistance has reduced the effectiveness of these chemicals and thus alternative control measures are required.
Phytophthora also effects commercially important crops and is responsible, among other things, for crown rot of apples. Again, while commercially available chemicals have been used to address problems associated with Phytophthora, there is a need for an improved substitute for currently available agents which are generally only partially effective against this family.
Silver-leaf is another plant disease for which currently available methods are only partially effective. The four general diseases (Botrytis, Armillaria, silver-leaf and Phytophthora) are characterised in that they all affect commercially important crops, and are difficult to control using commercially available agents, which are generally chemical based fungicides.
It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.